The fourth wall is a convention in the fiction medium which separates the action from the audience. This term has its roots in the 1800s, and it refers to the effect created in a proscenium theater, in which members of the audience sit on the other side of a "wall" created by the proscenium arch, looking into the set. Works of written fiction usually include a fourth wall, as do plays, movies, and television shows in which the fiction is dramatized.
By convention, the fourth wall is not mentioned or disturbed in the course of the events depicted. The maintenance of the wall helps the audience to suspend its disbelief, as fantastical events can happen beyond the wall but be perceived as acceptable, since they appear in the isolated universe of the fiction itself. In the case of dramatized fiction, the actors usually play to the fourth wall, and the set is oriented towards this imaginary barrier to ensure that the audience can clearly see what is happening.
In some works, people transcend the boundaries of the fourth wall. This practice is known as "breaking the wall," and it can be jarring or unsettling to an audience. A classic example of this is an aside or narration which is directed at the audience, as for example when a character in a Shakespeare play steps to the side of the stage and embarks upon a soliloquy; the other characters do not hear or respond to the speech, because the speech is directed through the fourth wall.
Breaking the wall can be an excellent narrative device, with many artists using the technique to jar, startle, or amuse the audience. A number of television programs have used a documentary-style device, allowing characters to speak directly to the camera as if being interviewed. When done poorly, however, going through the fourth wall can ruin the look and feel of the piece, making the audience feel alienated or confused. In a movie, for example, if a character abruptly begins to address the camera, it might feel unbalanced and strange.
People can also break through the fourth wall within the context of a narrative. In The Matrix, for example, the hero realizes that he is living in a fictional world, and he pulls himself out of that world and into the "real" world of the narrative. Many works of fiction have used this device, unpeeling layers of reality like an onion. In some cases, the audience may be in on the deception, and in other instances, the revelation of an entirely different reality may come as a surprise.